I'm sure the answer to this question is, "it depends" but I'm hoping for a ballpark number here.
Let's assume you design a smallish satellite to point at the earth while orbiting. You have the ability to dump momentum to a magnetorquer, no problems there. What I'm curious is, how often does that process usually occur? Is that something that you can get the rotation of the satellite pretty close and only dump the wheels once a month? Or does that process usually need to happen more often due to an accumulation of other forces acting on the satellite?